Huh, I've never heard of SoftMaker Office before, good to know it exists. I might check it out.

To add to some of the other comments, I have heard that the issue for LibreOffice is that Microsoft's own parser isn't compliant with the OOXML standard that they created. Yet the most important thing is compatibility with Microsoft Office, so you can't simply build a parser according to the open standard and expect it to work with Microsoft Office. Instead, you need a parser to work the same way as Microsoft's, which is proprietary. However, admittedly I have never read the OOXML standard or checked MS Office documents for compliance myself.

Therefore, if what I have heard is correct, I would assume that SoftMaker Office has either struck a deal with Microsoft before to improve compatibility, or has simply been better at reverse engineering. Alternatively, what I have heard could be wrong.